Treatment of choice for advanced disease is 5-FU, however response rate to this therapy is low. Recent reports detail increased cytotoxicity against a human colorectal carcinoma cell line with combined 5-FU & AZT. This trial of AZT & 5-FU will assess concomitant administration of these agents, & fully detail the metabolic & pharmacokinetic profile of AZT before evaluating combined AZT & 5-FU.